Set Resolution Automatically on Video File Load
I asked for this Auto-Dimensions-changer about same time that the time remapping occurred. IIRC the issue was knowing what the dimensions of the media was before importing it? Anyway the bottom line was that it couldn't be done at import, however there is a retrospective change you can make after setting a clip on the timeline.

Select the strip and go to the Strip menu > Set Render Size. This will alter the current scene's dimensions to match the source clip.